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Re: JULDAY 5.4 not same as 5.3? [message #24046 is a reply to message #23992]

Mon, 05 March 2001 14:57

Mark Hadfield
Messages: 783
Registered: May 1995

Senior Member

"Craig Markwardt" <craigmnet@cow.physics.wisc.edu> wrote in message
news:onzoezonz8.fsf@cow.physics.wisc.edu...
> pit@phys.uu.nl (Peter Suetterlin) writes:
> ...
>> I'm still not sure who's to blame, me (i.e. the programmer) or
>> IDL/RSI, but in general it does not make sense to take a negative of a
>> unsigned number, and IMHO there should also occur an automatic type
>> conversion as soon as a minus sign is involved. Currently, you get e.g.
>>
>> IDL> x=5b
>> IDL> print,-x
>> 251
>
> We can argue all day about what is correct mathematically.
>
> However I think it is correct from a microprocessor standpoint (ie,
> that's what the processor does). Further more it satisfies certain
> identities like:
>
> (-x) + x EQ 0

Actually, under Peter's proposal (-x) + x would equal 0 for unsigned x.

> And what would you do with a number like -('ffffffff'xul), ie a number
> that to begin with is too large to fit into a signed type.

Now, that is a good point.

There are other situations where auto-magical type conversion could save
programmers' skins but is not done by IDL, e.g. 32767S + 1S evaluates
to -32767S, not 32768L. Hands up who hasn't been stung by that one!

On the whole I agree with Craig that type conversion should be done
sparingly by the core language, because it opens up a can of worms. If -5B
is promoted to an unsigned integer, what about (0B-5B)? If the latter is
promoted, then what about (6B-5B)?

---
Mark Hadfield
m.hadfield@niwa.cri.nz http://katipo.niwa.cri.nz/~hadfield
National Institute for Water and Atmospheric Research

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[Message index]

		JULDAY 5.4 not same as 5.3? By: Don Woodraska on Thu, 01 March 2001 15:00
		Re: JULDAY 5.4 not same as 5.3? By: pit on Tue, 06 March 2001 09:23
		Re: JULDAY 5.4 not same as 5.3? By: Mark Hadfield on Mon, 05 March 2001 14:57
		Re: JULDAY 5.4 not same as 5.3? By: Craig Markwardt on Mon, 05 March 2001 14:22
		Re: unsigned int woes (was: JULDAY 5.4 not same as 5.3?) By: Martin Schultz on Mon, 19 March 2001 06:19
		Re: JULDAY 5.4 not same as 5.3? By: pit on Mon, 05 March 2001 10:03
		Re: JULDAY 5.4 not same as 5.3? By: Mark Hadfield on Sun, 04 March 2001 12:14

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